In recent years, systems such as power smoothing systems based on wind power generation, midnight power storage systems, household dispersive power storage systems based on solar power generation technologies, or power storage systems for electric vehicles have attracted attention from the viewpoint of effective utilization of energy aiming at protection of the global environment and conservation of resources.
The first requirement of a battery used in these power storage systems is high energy density. Research is actively proceeding on the development of lithium ion batteries as leading candidates for use as high energy density batteries capable of meeting such requirements.
The second requirement is high output characteristics. For example, when combining a highly efficient engine and a power storage system (as in, for example, a hybrid electric vehicle) or combining a fuel cell and a power storage system (as in, for example, a fuel cell electric vehicle), high output discharging characteristics are required by the power storage system during acceleration.
At present, devices such as electric double layer capacitors and nickel-metal hydride batteries are being developed for use as high output power storage devices.
Electric double layer capacitors using activated carbon for the electrodes have output characteristics of about 0.5 to 1 kW/L. These electric double layer capacitors also have high durability (cycle characteristics and storage characteristics at high temperatures) and have been considered to be the most suitable device in fields where high output is required. However, the energy density thereof is only about 1 Wh/L to 5 Wh/L. Consequently, further improvement of energy density is required.
On the other hand, nickel-metal hydride batteries, which are currently employed in hybrid electric vehicles, have a high output equivalent to that of electric double layer capacitors and have an energy density of about 160 Wh/L. However, research is actively proceeding in order to further enhance the energy density and output thereof together with enhancing durability (particularly with respect to stability at high temperatures).
In addition, research is also proceeding with the aim of increasing the output of lithium ion batteries. For example, a lithium ion battery has been developed that allows the obtaining of a high output of 3 kW/L at 50% depth of discharge (value indicating the state to which a power storage element has discharged as a percentage of the discharge capacity thereof). However, the energy density thereof is equal to or lower than 100 Wh/L, i.e., it is designed to intentionally suppress high energy density, which is the greatest characteristic of lithium ion batteries. In addition, the durability (cycle characteristics and storage characteristics at high temperatures) thereof is inferior to that of electric double layer capacitors. Consequently, these batteries are limited to use over a range of depth of discharge that is narrower than 0 to 100% in order to maintain practical durability. Since capacitance at which the battery is actually able to be used is even lower, research is actively proceeding to further improve durability.
As was previously described, there is a strong demand for the practical application of power storage elements provided with high energy density, high output characteristics and durability. However, each of these existing power storage elements has its merits and demerits. Consequently, a new type of power storage element is required that satisfies these technological requirements. Power storage elements referred to as lithium ion capacitors have attracted attention as a leading candidate for satisfying these requirements and are currently being actively developed.
Capacitor energy is expressed as ½·C·V2 (where, C represents capacitance and V represents voltage).
Lithium ion capacitors are a type of power storage element (nonaqueous lithium-type power storage element) that use a nonaqueous electrolytic solution containing a lithium salt, and carry out charging and discharging by a non-Faraday reaction based on adsorption/desorption of anions in the same manner as electric double layer capacitors at roughly equal to or higher than 3 V at the positive electrode, and by a Faraday reaction based on intercalation/release of lithium ions in the same manner as lithium ion batteries at the negative electrode.
In summarizing the aforementioned electrode materials and characteristics, although high output and high durability are realized in the case of carrying out charging and discharging by adsorption/desorption of ions on the surface of activated carbon (non-Faraday reaction) using a material such as activated carbon for the electrodes, energy density is low (such as being only one-fold). On the other hand, although energy density increases (such as increasing to 10-fold that of non-Faraday reactions using activated carbon) in the case of carrying out charging and discharging by a Faraday reaction using an oxide or carbon material for the electrodes, there are problems with durability and output characteristics.
Electric double layer capacitors are characterized in that they combine these electrode materials by using activated carbon for the positive electrode and negative electrode (one-fold energy density) and carrying out charging and discharging by a non-Faraday reaction at both the positive and negative electrodes, thereby demonstrating the characteristics of high output and high durability but low energy density (one-fold at the positive electrode×one-fold at the negative electrode=1).
Lithium ion secondary batteries use a lithium transition metal oxide for the positive electrode (10-fold energy density) and a carbon material for the negative electrode (10-fold energy density), and are characterized by carrying out charging and discharging according to a Faraday reaction at both the positive and negative electrodes, and although these batteries demonstrate high energy density (10-fold at the positive electrode×10-fold at the negative electrode=100), they have problems with respect to output characteristics and durability. Moreover, the depth of discharge must be restricted in order to satisfy the high durability required by applications such as hybrid electric vehicles, thereby resulting in lithium ion secondary batteries only being able to use 10% to 50% of the energy thereof.
Lithium ion capacitors are characterized by using activated carbon for the positive electrode (one-fold energy density) and using a carbon material for the negative electrode (10-fold energy density), and carrying out charging and discharging by a non-Faraday reaction at the positive electrode and by a Faraday reaction at the negative electrode, enabling these capacitors to function as novel asymmetrical capacitors provided with both the characteristics of electric double layer capacitors and lithium ion secondary batteries. These lithium ion capacitors have high energy density (1-fold at the positive electrode×10-fold at the negative electrode=10) while still retaining high output and high durability, and are characterized by not requiring restriction of depth of discharge in the manner of lithium ion secondary batteries.
Examples of applications that use lithium ion capacitors include power storage for railways, construction machinery and automobiles. In these applications, the capacitor used is required to have superior temperature characteristics due to the harsh operating environment. In particular, decreased performance caused by the generation of gas attributable to decomposition of the electrolytic solution at high temperatures is a problem. A counter technology for this problem consists of adding an additive to a nonaqueous electrolytic solution to form a coating film composed of decomposition products thereof on the surface of the negative electrode active material, thereby inhibiting reductive decomposition of the nonaqueous electrolytic solution accompanying subsequent charging and discharging and improving battery durability. Related technologies in PTL 1 and PTL 2 propose a power storage element containing two types of additives having different structures in an electrolytic solution. In addition, PTL 3 proposes a power storage element in which a fixed amount of a coating film is formed on the surface of the negative electrode active material by adding an additive.
In addition, there is the potential for decreases in performance and internal short-circuiting caused by precipitation of lithium dendrites at the negative electrode interface as a result of using in low temperature environments at 0° C. or lower, thereby resulting in significant problems in terms of safety and reliability of the power storage element.
PTL 4 proposes a means for solving such problems with a lithium ion capacitor having improved low temperature characteristics by containing a specific solvent in the electrolytic solution.